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(1) Field of the Invention
This invention relates to a high-speed laser-based via generation system for producing through-substrate and blind vias to make interconnects in high-density microelectronics systems. More particularly, this invention relates to an opto-mechanical system which delivers controlled pulses of laser energy to a large number of program-selectable via sites simultaneously at very high optical efficiency regardless of the number or density of vias. The system delivers the full energy of the laser among the vias being generated through the use of a high-speed opto-mechanical beam-steering system and a specialized energy recycling illumination system.
(2) Description of Related Art
Vias, the small diameter holes through one or more substrate layers, play an important role in electronic manufacturing because they provide the interconnections between layers in electronic modules. Electronic manufacturing has evolved to provide denser, faster, and more complex packaging technologies which rely heavily on a large number of micro-viasxe2x80x94vias with diameters less than about 150 xcexcmxe2x80x94to provide the increasing number of required interconnections. The generation of these vias in electronic substrates has become a throughput-limiting and crucial step in the fabrication of advanced electronic modules. Via numbers and densities in these modules have increased to keep pace with the increasing complexity of electronic devices, but current via generation technologies cannot keep up with this growth and are fundamentally limited with respect to achievable throughput rates. This invention seeks to address this problem.
Depending on the application, the vias may need to be generated in regular, periodic patterns, such as in chip carriers, or in non-periodic patterns, such as on high density interconnect printed wiring boards. Often the vias need to be drilled through one or several substrate layers. Almost as often, the vias need to be generated such that they terminate or bottom out at a particular layer or depthxe2x80x94these are called blind vias. The vias need to be generated in a wide range of electronic substrate materials. These materials are most commonly dielectric polymers or glass-filled epoxies, but can also be more mechanically durable materials such as ceramics and metals. The wide range of substrate material properties poses a challenge for via generation systems.
Several technologies currently exist for via generation in electronic substrates: conventional mechanical drilling using metal or ceramic bits; lithographic patterning of the via pattern followed by chemical or plasma etching of the substrates; and direct laser photo-ablation. The mechanical technologies cannot produce micro-vias at high throughputs or with fine dimensions due to breakage of the bits and minimum diameter limitations. Lithographic and etching technologies are expensive, require multiple processing steps, are difficult to reconfigure, and are hard to control in depth for blind via applications. Optical technologies using laser photo-ablation show the most promise for rapid, clean production of micro-vias in a variety of substrates, but current systems are too slow to keep up with current production demands. This is mainly due to the serial nature of the via generation process and the power limitations of current frequency multiplied solid-state laser systems.
Laser via generation through photo-ablation provides many advantages: it allows a multitude of materials to be drilled without generation of harmful debris; it allows flexibility in via diameter, depth, and placement; and it typically does not require additional process steps. Photo-ablation is the process by which material is broken down to its smaller molecular or elemental components by being irradiated by a very high-fluence beam of ultraviolet radiation. For most materials photo-ablation is most efficient for radiation in the UV-region of the spectrum since most materials absorb energy very strongly at these wavelengths. For each material and laser wavelength there is a photo-ablation threshold fluence above which ablation occurs efficiently, and there is a photo-etch rate that characterizes the rate at which the material is ablated away. For most dielectric materials used in electronics manufacturing the ablation threshold fluence can range from about 100 mJ/cm2 to several Joules/cm2 at wavelengths near 300 nm, and etch-rates for 1 J/cm2 fluences at these wavelengths are typically 0.5 xcexcm/pulse. For this reason, powerful UV lasers are required to generate vias efficiently. Currently, only excimer lasers such as XeCl and certain frequency-multiplied solid-state lasers such as Nd:YAG can deliver the required fluences. Excimer lasers are attractive because they are the most powerful of these, whereas frequency-multiplied solid-state lasers are used due to their high repetition rates and focusability.
Systems which use solid-state laser sources typically focus the low average power, frequency-multiplied beam into a very small spotxe2x80x94typically about 20 xcexcm in diameterxe2x80x94to achieve the high fluences required for photo-ablation. They generate the required via patterns by a combination of galvo-mirror beam deflection to move the spot on the substrate and automated planar (X-Y) stage motion to present different areas of the substrate to the beam. The throughput of these systems depends on how quickly the focused laser spot can be moved from one via site to the next, by the dwell time required at via site, and by the average power of the laser source. Although this process can be easily programmed and is, thus, very flexible, it is essentially a serial process, and therefore, has an inherently low throughput. In addition, via diameters are limited to a minimum size of about 20 xcexcm.
Systems using scanning mask projection and high-power excimer lasers for massively parallel via generation have been described in Jain, U.S. Pat. No. 5,285,236. Such systems capitalize on the much greater powers delivered by excimer lasers to generate the required fluences over large areas of the substrate. In these systems a large-area, uniform beam is produced by a specialized illumination system, as described in Jain, U.S. Pat. No. 5,059,013, and Farmiga, U.S. Pat. No. 5,828,505. This beam illuminates a via pattern on a mask which is projected onto the substrate by a projection imaging system. In such a system, all the vias in the illuminated region are generated simultaneously, the throughput being limited only by the etch-rate of the material and not the number of vias. For very dense via density applications, this type of system can achieve extremely high throughputs, especially when the illumination system incorporates energy recycling as described in Hoffman and Jain, U.S. Pat. No. 5,473,408. For low via densities, however, such a large-area projection system can be slow and inefficient. In addition, such a system is not very flexible in that it requires expensive masks to be generated for each required via pattern.
Current electronic manufacturing demands via generation systems with the programmability of the serial solid-state laser-based systems and the high throughputs of the massively parallel excimer laser-based systems. Highly desirable features are high-speed via generation; full via pattern programmabilityxe2x80x94including via diameter, position, and depth; capability to drill high-threshold photo-ablation substrates; and full and efficient utilization of available laser energy. The invention described below provides all these features. It makes full and efficient use of the power available from excimer lasers and provides full programmability of the via pattern.
This invention is a via generation system which produces vias in a variety of microelectronic substrate materials by the process of laser photo-ablation. This system optimally utilizes the full energy available from the laser source by efficiently dividing and channeling the radiation into a number of beams which is exactly the same as the number of vias to be generated in a given module region. The system ablates all vias in a particular module region simultaneously and allows excess energy (i.e., the unused beam energy due to absence of a via at a location) to be xe2x80x98recycledxe2x80x99 to speed the process and make it more efficient. A computer control system allows each via location, size, and depth to be fully programmed and automatically generated. The various subsystems, objects, features, and advantages of this via generation systems are described in more detail below.
An excimer laser, such as a high-power xenon chloride laser operating at 308 nm, serves as the radiation source for the system. The output beam from this laser is shaped and uniformized by the illumination subsystem, a system of optics including standard lens elements and a xe2x80x98homogenizerxe2x80x99 unit. The homogenizer unit includes a special mirror at its input end that allows reflected radiation from further down the optical train to be returned into the train for more efficient utilization, in effect recycling the reflected energy. The output of the illumination subsystem is an approximately collimated beam with excellent intensity uniformity and the proper cross-sectional shape, typically hexagonal to allow for densest packing of via-generating beamlet grid as will be described below. The output beam is sized to obtain a high enough fluence at the substrate for photo-ablation; larger beams allow higher throughputs, but the fluence drops as the area is increased.
The shaped, uniformized, and collimated laser beam enters the beamlet steering subsystem which directs the radiation to the proper via sites. The beamlet steering subsystem is a high-speed opto-mechanical system that divides the incident beam into many beamlets and allows computer-controlled steering of each individual beamlet produced to a selected via site.
In the first embodiment of the invention, the nearly collimated input beam from the illumination subsystem is divided into an array of beamlets by a two-dimensional array of micro-lenses known as a fly""s-eye lens array. Each lenslet in the array focuses a small fraction of the incident beam onto a single, corresponding, tip-tilt mirror element in a computer-controlled two-dimensional tip-tilt mirror array. The mirror elements in this array are actuated by high-speed actuators such as piezo-ceramic pistons working in groups to accomplish the required tip-tilt motion. The computer control specifies the tip and tilt angles of each mirror element, and, thus, the direction in which each beamlet will be reflected. In this way, computer-controlled simultaneous steering of a large number of beamlets is realized. The beamlets are directed by the steering mirror array into a large array of optical fibers which channel the radiation to the substrate either directly (close proximity) or by a projection lens (which allows a large working distance). Each steering mirror element can direct its beamlet into one of several fibers within its steering range. Typically one mirror element can address any one out of seven fibers packed in a dense, hexagonal array. Beamlets which are not required for ablation (if the number of vias being patterned is less than the maximum), can be reflected back into the energy-recycling illumination subsystem which reuses the available radiation. The optical fiber array covers a certain region of the substrate and each fiber can deliver radiation for photo-ablation of the substrate at one location in this region. Thus, radiation is directed to any of a large number of sites on the substrate for via generation. Via target sites which do not fall on the grid formed by the fiber array are accessed by moving the substrate and/or beam-steering system with a precision X-Y stage. The final result is that a large number of beamlets can be efficiently directed to many different via locations on the substrate simultaneously to generate the required via pattern.
In the second embodiment of the invention, a lens system placed after the homogenizer images the input face of the homogenizer to produce a focal spot array which then impinges on a two-dimensional mirror array similar to that described above. This focal spot array (FSA) is formed because the point source of radiation entering the homogenizer undergoes a multitude of reflections within the internally mirrored homogenizer unit and, when imaged, appears as a multitude of point sources corresponding to the number and angle of the reflections produced within the homogenizer unit. The homogenizer and FSA lens are designed to produce the grid of focal spots required. The intensity of the various focal spots in the FSA can be equalized by the use of an apodization filter if required. Each spot in the FSA impinges on a single steering mirror element and is reflected into a projection lens which refocuses the radiation tightly onto the substrate. By controlling the tip and tilt of the mirror elements, each spot can be positioned anywhere in the image field of the projection lens to generate the required via patterns. Multiple beamlets from different mirror elements can be directed to the same via site to speed the ablation process, or unneeded beamlets can be reflected back into the illumination system for energy recycling.
In either embodiment, the computer control system is a critical component as it controls the beam-steering mirror array to position the beamlets at the correct via locations on the substrate and coordinates the other subsystems to generate the vias correctly. The control system reads information on the via patterns from standard CAD filesxe2x80x94this information includes via locations, sizes, and depthsxe2x80x94and coordinates the firing of the laser, the motion of the X-Y stages, and, most importantly, the tip-tilt beamlet steering mirror arrays to generate the correct via patterns on the substrate. Because all aspects of the beamlet delivery can be controlled, optimized patterning algorithms can be generated to maximize the efficiency of the via generation process.
For each module being covered on the substrate at a given moment the number and location of beamlets needed to address the via sites selected for via formation will be known in advance from the CAD file. The total laser energy required for full ablation, which can be expressed as the number of laser pulses required to drill through the substrate, depends on the number of selected via sites in the module. Since a portion of the incident laser radiation is recycled when the number of vias is less than the maximum, fewer pulses are required to drill this smaller number of via sites. The varying energy requirement from module to module is managed by the control system to ensure that the correct number of laser pulses, and hence energy, is delivered to each site regardless of how many via sites the module contains. This allows a highly efficient use of the available laser power without sacrificing the benefits of massively parallel via generation.
The object of the invention is to provide an efficient system for laser formation of vias in microelectronics substrates over a very wide range of via densities.
A more specific object of the invention is to provide a computer-controllable pattern of high-fluence, shaped, spatially homogenized, pulse-repetition-controlled, and directed laser beamlets at the selected sites of the substrate to form by photo-ablation the desired via pattern in minimal time with optimal efficiency.
A feature of the invention is energy recycling together with computerized control of the number of laser pulses as required for the number of vias patterned in a given module region. In this way, all the vias in a given module region are generated simultaneously with optimal energy utilization, greatly increasing throughput over serial via generation systems and also over mask-projection via generation systems tasked with patterning modules of varying via densities.
A feature of the invention is the pairing of a fly""s-eye micro-lens array with a computer-controlled tip-tilt micro-mirror array to steer an array of beamlets into grouped optical fibers. Each pair of a fly""s-eye micro-lens and a micro-mirror can deliver a beamlet to one fiber in an optical fiber group which is part of a larger fiber array, e.g., to one of seven optical fibers grouped six-around-one in a tight hexagonal grouping. Each fiber thus addressed channels the radiation to a unique via location on the substrate allowing a via to be generated there by photo-ablation.
Another feature of the invention is the pairing of a focal spot array generating optical system with a computer controllable tip-tilt micro-mirror array to position beamlets through a projection lens onto selected via sites on the substrate, thereby generating the desired via pattern by photo-ablation.
Another feature of the invention is computer control of the number of laser pulses applied to a selected substrate module region as a combined function of number of vias to be drilled and calculated energy recycling.
Still another feature of the invention is selective fine positioning of the vias within the substrate module by computer controlled displacement, jittering, overlapping, rotating, or other motion of the substrate stage or beamlet steering optics as required to allow for via generation at any set of points on the substrate. This allows vias to be generated with a closer spacing than would be possible with mask-based systems.
An advantage of the invention is that the via generating process can be optimized for each module region of the substrate (for high throughput) by full control of the applied irradiation. The control computer can calculate the optimal number of pulses, beamlet positions, and X-Y translation schedule based on the desired via pattern and the energy recycling factor.
Another advantage of the invention is the ability to use recycled radiation energy to increase the rate of generation of vias in low- to medium-density patterns.
Another advantage of the invention is that the same system may be efficiently used for generating via patterns over a large range of via densities range on the same substrate.